Tumor-associated Natural Killer cells in ovarian cancer ascites: Molecular and functional characterization
The high-grade serous carcinoma (HGSOC) is the most common ovarian cancer type that, due to late diagnosis, has the highest mortality rate of all gynecological cancers. One third of the patients develop a fluid in the peritoneal cavity called ascites. The ascites contains tumor and immune cells as w...
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|The high-grade serous carcinoma (HGSOC) is the most common ovarian cancer type that, due to late diagnosis, has the highest mortality rate of all gynecological cancers. One third of the patients develop a fluid in the peritoneal cavity called ascites. The ascites contains tumor and immune cells as well as soluble immunomodulatory factors such as cytokines.
NK cells - as a crucial part of the innate immune system - play a major role in tumor immune surveillance. Their effector function is tightly regulated by the balance between activating and inhibitory receptor signaling. However, NK cell function of many tumor patients is restricted and the overall results of NK cell-based immunotherapy are unsatisfying. This is often due to tumor-associated immune escape mechanisms.
One of the best characterized activating receptors of NK cells is NKG2D, which recognizes malignant cells and mediates tumor cell lysis. However, in various tumor entities, the NKG2D/NKG2D-Ligand (NKG2D-L) signaling axis is defective. Shedding of NKG2D-Ls from the surface of tumor cells to a soluble form is one such evasion strategy. The interference mechanisms of the ligands include (1) the downregulation of NKG2D and (2) passive blocking of NKG2D. Subsequently, this leads to dysfunctional NK cells and facilitated tumor growth. However, in mice, (3) they can also inhibit the NKG2D/NKG2D-L macrophage – NK cell crosstalk and thereby support NK cell-mediated tumor cell lysis. The relevance of this mechanism in humans is elusive.
In ovarian cancer ascites, soluble NKG2D-Ls are enriched and negatively correlate with the survival of the patients. Surprisingly, the expression of NKG2D on NK cells and T cells is not reduced. In this thesis, I could show that human macrophages did not express NKG2D excluding a direct interaction of NKG2D-Ls with macrophages. However, NKG2D-Ls were detectable on the surface of macrophages, but the killing of macrophages by NK cells was not NKG2D-dependent. This indicates that, in ovarian cancer, soluble NKG2D-Ls do not influence the NKG2D signaling through the mechanisms described so far in other models. Instead, I provided evidence that soluble NKG2D-Ls have an additional function – namely the induction of NKG2D signaling and target gene expression upon engagement.
Furthermore, not much is known about the phenotype and differentiation of healthy NK cells to tumor-associated NK cells (TANKs) and in particular about signaling pathways that are involved. In this thesis, I showed that patient-derived NK cells depicted a mixed tumor-associated phenotype, in which NK cells displayed a strong downregulation of most of the activating receptors and an upregulation of the inhibitory receptors PD-1 and TIM-3, indicating a state of exhaustion. These NK cells showed impaired NK cell effector functions, which could be restored upon cytokine activation and blockage of the TGFβ signaling pathway. Moreover, TANKs showed a dysregulation of the natural killer cell-mediated cytotoxicity and the Hippo pathways. I established that proteins suppressed by the Hippo pathway, such as YAP1 and TEAD4, were exclusively expressed in TANKs, while they were absent in healthy donor NK cells. However, if healthy donor NK cells were stimulated with TGFβ, IL-2 and via CD16 crosslinking, expression of TEAD4 was induced. TEAD4-expressing NK cells showed a TEAD4-dependent upregulation of TIM-3. Finally, I provided first evidence that TEAD4-expressing NK cells could acquire a regulatory function, in which they were able to suppress T cell proliferation.
These findings shed more light on the role of NK cells in the tumor microenvironment (TME) of ovarian cancer patients. Based on that, further research will increase our knowledge of NK cell tumor surveillance, paving the way for strategies boosting anti-tumor NK cell functions.